1. Field of the Invention
The present invention relates to a coating apparatus, particularly, to a spinner type coating apparatus in which a surface of an object to be coated, which is disposed on a spin chuck, is coated with a process solution.
2. Description of the Related Art
FIG. 1 shows a conventional resist coating apparatus. As seen from the drawing, the resist coating apparatus comprises an annular cup 200. A spin chuck 202 is arranged in the central portion within the annular cup 200. A semiconductor wafer W is disposed on the spin chuck 202. Under this condition, a resist solution is dripped through a resist nozzle 204 onto the surface of the semiconductor wafer W. During the dripping, the spin chuck 202 is rotated so as to rotate the semiconductor wafer W, with the result that the resist solution is centrifugally expanded such that the entire wafer surface is uniformly coated with the resist solution.
The resist solution scattered from the semiconductor wafer W during the resist coating step runs to impinge against an upper portion of the inner wall of the annular cup (drain cup) 200 and, then, is guided to the bottom of the cup 220, as denoted by a solid line R. Further, the scattered resist solution flows through a waste solution outlet port 200a and a pipe 206 into a waste solution tank (not shown). As seen from the drawing, a discharge path 200b of a labyrinth structure formed by an intermediate hanging wall 201 and an intermediate upright wall 203 is arranged within the cup 200. The discharge path 200b functions as a mist trap. To be more specific, mist which is converted into an exhaust gas is removed from the scattered resist solution within the discharge path 200b, as denoted by a solid line G, and, then, guided into a discharge chamber 200c on the side of the inner circumferential surface of the annular cup 200. Further, the mist guided into the discharge chamber 200c is further guided into an exhausting pump (not shown) through an outlet port 200d of the discharge chamber 200c and a pipe 208.
As described above, the conventional apparatus is constructed such that the scattered resist solution is separated within the drain cup 200 into the waste solution and the exhaust gas which are discharged through the outlet ports 200a and 200d, respectively, which are formed separately from each other. What should be noted is that the mist removed from the scattered resist solution, which is guided to the discharge path 200b together with the exhaust gas, is attached to the surfaces of the intermediate hanging wall 201 and the intermediate upright wall 203. Then, the mist attached to the wall surfaces is solidified after drying so as to cause plugging of the discharge path 200b acting as a mist trap. To prevent the plugging problem, required is a troublesome operation of frequently washing the discharge path 200b with a solvent such as a thinner.
Also, an on-off control alone is applied to the gas discharge system in the conventional apparatus. If a gas is kept discharged during the processing, the peripheral portion of an object to be processed is solidified relatively faster than the central portion, resulting in failure to form a film of a uniform thickness on the surface of the object. Further, if a gas is kept discharged during non-processing, a process solution such as a resist solution is dried and solidified in the gas discharge system, making it necessary to wash the apparatus before restarting of the processing. If a gas discharge is stopped during the non-processing, however, the gas discharge system is filled with an evaporated solvent, leading to generation of offensive odor. In addition, since the evaporated solvent tends to catch fire easily, it is necessary to pay careful attentions in restarting the processing. Particularly where it is necessary to stop operating the apparatus for a long period of time, the problems pointed out above are rendered more serious.